Necessity of Thermostat

I was in a truck stop a few hours ago buying diesel for my 2500HD duramax.
Another pickup pulled up next to me with smoke billowing out of the engine compartment. It was an F350 dually diesel. The driver told me
he had developed a radiator leak a few weeks ago and began to over heat. His solution was to remove the thermostat. My response was for him to visit the nearest repair shop immediately. Apparently today, his engine began to over heat so badly that smoke billowed from the engine. Apparently, the engine oil was smoking.
Hopefully he will get it fixed before the engine blows.
In any case, my question is concerning the thermostat. Personally, I would never operate my truck without one. But, the other driver's one comment made me wonder. What does the thermostat do? I am sure it closes off coolant flow to the radiator to allow the engine to warm up faster and to prevent the engine from running too cool.
But what about the other end? It seems that a missing thermostat (removed) is identical to one stuck open which would allow constant coolant flow through the radiator. In this case, it would seem that maximum cooling capacity is provided. If my idea is correct, why would an engine overheat without a thermostat?
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David wrote:

Because the thermostat also provides a restriction to coolant flow that allows the coolant to absorb the heat better. This is also why the thermostat is the last item in line to the radiator.
Another problem is the people who run a 160 thermostat in a vehicle designed for a 195. The 160 will constantly make the computer think the engine is cold and will richen the mix up to compensate. Constantly running this way can burn out the converter and kills your gas mileage as well.
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Steve W.
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"David" wrote in message

Actually, you should never operate your truck without two! I replaced the thermostat on my '06 3500 Duramax last fall and found that there are two thermostats, a front and a back and they aren't interchangeable. I also found out they are only sold in sets.
Ron
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